The present invention relates to noise detection circuits, and more particularly to circuits for rejecting noise signals which occur randomly and outside of a predetermined time pattern which is known to be representative of the time of occurrence of desired signals. The invention has particular application in conjunction with radiation detection tubes, which generate electrical signals upon receipt of incident radiation over predetermined light wavelengths. Such radiation detection tubes characteristically generate random noise signals over widely-spaced time intervals, such signals having no relation to incident light or electromagnetic radiation, but are thought to be caused by internal physical reactions within the radiation detection tube.
Among the uses of radiation detection tubes in industrial applications is the use of such tubes to detect incident electromagnetic radiation over predetermined band widths characteristic of radiation band widths caused by fire or flame. Such detection devices have particular utility as fire detectors, for their output signals can be closely coordinated with predetermined band widths of radiation which are associated with flame radiation. If radiation detection tubes for this application are properly designed and selected, they will generate output signals upon receipt of incident flame radiation, but will reject incident radiation outside of a predetermined narrow band width and as a consequence of such extraneous incident radiation will generate no substantial output signals. A characteristic of these devices is that the repetition of output signal generation is directly related to the intensity of a flame, wherein an initial start of a fire will cause the generation of a few closely spaced signals, and as a fire increases in intensity the number of generated signals increase in frequency.
On the other hand, such detection devices suffer from the disadvantage that internal physical reactions tend to generate occasional and random signals, such signals being totally unpredictable as to time of occurrence, but generally sufficiently widely spaced in time so as to be recognizable as "false" signals. In any application where such a radiation detection tube is utilized as the flame detecting device, it is therefore important to discriminate between "false" signals and signals which are truly representative of a detected flame. This is particularly true in industrial applications wherein a flame detector may be utilized as a sensing element to set off a fire extinguishing system, including water sprinklers, chemical fire extinguishers, or other fire retarding measures. If such devices are set off by random noise signals they will usually cause great and unwarranted damage to the area being monitored. It is therefore important that such fire detection systems reliably detect the existence of a real flame, but equally reliably reject the occasional noise signals which are inherently generated within radiation detection tubes, so as to initiate fire extinguishing and other emergency measures only when absolutely necessary.